1. Field of the Invention
The present disclosure relates to secondary-battery protecting integrated circuits, secondary battery protection apparatuses, and battery packs.
2. Description of the Related Art
Conventionally, a circuit for protecting a secondary battery is known, in which the protection operation is performed by turning off a discharge FET (Field-Effect Transistor) disposed between a negative electrode of the secondary battery and a negative terminal connected to ground of a load (e.g., Japanese Unexamined Patent Application Publication No. 2012-257407). In this circuit, a terminal for inputting a control signal is provided. A state of the circuit is transitioned into power down state and the discharge FET is turned off in response to the control signal being input at the terminal, thereby reducing power consumption in the secondary battery.
FIG. 1 is a circuit diagram illustrating an example configuration of a secondary-battery protecting integrated circuit 420 including a control terminal CNT at which the control signal is input. The secondary-battery protecting integrated circuit 420 is an example circuit in which a secondary battery 500 is protected by turning off a discharge control transistor 312 to prevent the secondary battery from being in an abnormal state in response to detecting an abnormality, such as over discharge. The discharge control transistor 312 and the charge control transistor 311 are connected in series between a negative electrode 502 of the secondary battery 500 and a negative terminal P− connected to ground of an electronic apparatus 430. Also, the electronic apparatus 430 is an example load having a resistance component 452. The secondary-battery protecting integrated circuit 420, the secondary battery 500, the discharge control transistor 312, and the charge control transistor 311 are integrated in a battery pack 400. The electronic apparatus 430 is connected to and powered by the battery pack 400 via a charge/discharge prevention switch 510.
A control signal, whose voltage varies according to ON state and OFF state of the charge/discharge prevention switch 510, is input at the control terminal CNT. When the charge/discharge prevention switch 510 is turned on, the voltage at the control terminal CNT increases. In response to a detection circuit 370 detecting that the voltage at the control terminal CNT exceeds a predetermined detection threshold of charge/discharge prevention, a control circuit 398 turns off the discharge control transistor 312 and the charge control transistor 311. Thus, charge/discharge operation of the secondary battery 500 is prevented. On the other hand, when the charge/discharge prevention switch 510 is turned off, the voltage at the control terminal CNT decreases. In response to the detection circuit 370 detecting that the voltage at the control terminal CNT is equal to or less than the predetermined detection threshold of recovery from charge/discharge prevention state, the control circuit 398 turns on the discharge control transistor 312 and the charge control transistor 311. Thus, the charge/discharge operation of the secondary battery 500 is allowed to be performed.
However, a pull-down resistor 373 may be disposed between the control terminal CNT and a ground terminal VSS, and a diode 451 may be parasitic or disposed as an electrostatic countermeasure between the control terminal CNT and the negative terminal P−. In this case, even if the discharge operation of the secondary battery 500 is prevented by turning off the discharge control transistor 312 (in discharge prevention state), a discharge current 437 from the secondary battery 500 may flow through a positive electrode 501, the resistance component 452, the diode 451, the control terminal CNT, the pull-down resistor 373, the ground terminal VSS, and the negative electrode 502.
When a resistance value of the pull-down resistor 373 is increased, increase of the discharge current 437 can be suppressed in the discharge prevention state. However, if the resistance value of the pull-down resistor 373 is increased (raised), the voltage at the control terminal CNT increases due to the discharge current 437. Therefore, recovery from the discharge prevention state may be unable to be performed even when the charge/discharge prevention switch 510 is turned off. That is, the detection circuit 370 cannot detect that the voltage at the control terminal CNT is equal to or less than the predetermined threshold due to the increased voltage at the control terminal CNT. Accordingly, the control circuit 398 may not turn the state of the discharge control transistor 312 and the charge control transistor 311 from OFF to ON.